Other assets - Favorable contracts (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
Dec. 31, 2018	Jul. 01, 2018	Dec. 31, 2019	Dec. 31, 2017
Finite-lived Intangible Assets [Roll Forward]
Amortization of favorable contracts	$ (58)	$ 0	$ (134)	$ 0
Favorable contracts
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	287		287	287
Accumulated Amortization	(101)		(254)	0
Finite-lived Intangible Assets [Roll Forward]
Net carrying amount, beginning		287	186
Amortization of favorable contracts		$ (101)	(153)
Net carrying amount, ending	$ 186		$ 33	$ 287

The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

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